From mboxrd@z Thu Jan 1 00:00:00 1970 From: Patrick McHardy Subject: Usage of skb->cb and dev->xmit_lock Date: Fri, 17 May 2002 01:58:53 +0200 Sender: owner-netdev@oss.sgi.com Message-ID: <3CE447BD.8040107@ws.localnet> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: To: netdev@oss.sgi.com List-Id: netdev.vger.kernel.org Hi. I'm looking for some advice concerning two things. First a little introduction: I've written a software device used as a placeholder for qdiscs to perform ingress shaping / shaping over multiple interfaces. It feeds itself packets marked by an iptables target through netfilter hooks. For marked packets NF_QUEUE is returned and the packet is then enqueued to the attached qdisc. The device's xmit function later reinjects them to the network stack. Now my problems: For reinjecting i need to keep the struct nf_info passed to my netfilter queue handler. I therefore introduced a new field in struct sk_buff. I don't really like this solution so i though of using skb->cb. It says "This is owned by whoever has the skb queued ATM." which of course if not the device but the qdisc. But i haven't found any refernece to it in any qdisc so my question is: is it safe to use skb->cb in this altered code flow ? The second problem arises when used with tunnels (only gre tested). If packets going to a gre tunnel are redirected though imq and the gre packets themselves are redirected too then qdisc_restart starts complaining about the tunnel device beeing deadlooped. (why the tunnel device and not mine?). I thought releasing dev->xmit_lock before calling nf_reinject and grabing it back afterwards would solve the problem, but i'm not sure about the consequences .. i guess on UP it doesn't matter, on SMP the worst thing i can imagine is one processor waiting until the lock becomes free for grabbing it back if in the meantime another processor has taken it. Can someone please enlighten me ? :) Thanks for your help, Patrick PS: If you want to look at the source, it is available at http://luxik.cdi.cz/~patrick/imq